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<h3><a name="SECTION0016349000000000000000"></a><a name="optparse-callback-example-6"></a>
<br>
14.3.4.9 Callback example 6: variable arguments
</h3>

<p>
Things get hairy when you want an option to take a variable number of
arguments.  For this case, you must write a callback, as <tt class="module">optparse</tt> doesn't
provide any built-in capabilities for it.  And you have to deal with
certain intricacies of conventional <span class="Unix">Unix</span> command-line parsing that <tt class="module">optparse</tt>
normally handles for you.  In particular, callbacks should implement
the conventional rules for bare <code>"-"</code> and <code>"-"</code> arguments:

<ul>
<li> 
either <code>"-"</code> or <code>"-"</code> can be option arguments

<p>
</li>
<li> 
bare <code>"-"</code> (if not the argument to some option): halt command-line
processing and discard the <code>"-"</code>

<p>
</li>
<li> 
bare <code>"-"</code> (if not the argument to some option): halt command-line
processing but keep the <code>"-"</code> (append it to <code>parser.largs</code>)

<p>
</li>
</ul>

<p>
If you want an option that takes a variable number of arguments, there
are several subtle, tricky issues to worry about.  The exact
implementation you choose will be based on which trade-offs you're
willing to make for your application (which is why <tt class="module">optparse</tt> doesn't support
this sort of thing directly).

<p>
Nevertheless, here's a stab at a callback for an option with variable
arguments:
<div class="verbatim"><pre>
def vararg_callback(option, opt_str, value, parser):
    assert value is None
    done = 0
    value = []
    rargs = parser.rargs
    while rargs:
        arg = rargs[0]

        # Stop if we hit an arg like "--foo", "-a", "-fx", "--file=f",
        # etc.  Note that this also stops on "-3" or "-3.0", so if
        # your option takes numeric values, you will need to handle
        # this.
        if ((arg[:2] == "--" and len(arg) &gt; 2) or
            (arg[:1] == "-" and len(arg) &gt; 1 and arg[1] != "-")):
            break
        else:
            value.append(arg)
            del rargs[0]

     setattr(parser.values, option.dest, value)

[...]
parser.add_option("-c", "--callback",
                  action="callback", callback=varargs)
</pre></div>

<p>
The main weakness with this particular implementation is that negative
numbers in the arguments following <code>"-c"</code> will be interpreted as
further options (probably causing an error), rather than as arguments to
<code>"-c"</code>.  Fixing this is left as an exercise for the reader.

<p>

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